1. Field of the Invention
The present invention relates to a light-emitting diode using an organic material and is applicable to a planar light source and the like.
2. Description of the Related Art
In recent years, organic light-emitting diodes (OLEDs) have been attracting attention in view of applications to a planar light source and the like. An OLED has a structure that an emission layer of a thin film comprising organic materials is interposed between two electrodes of a cathode and an anode. The OLED operates as follows: When a voltage is applied between the cathode and the anode, electrons and holes are injected to the emission layer from the cathode and the anode, respectively, and excitons are produced in the emission layer through recombination of electrons and holes. When the excitons are radiatively deactived, light emission is caused. The OLED utilizes the light emission. In particular, OLEDs using phosphorescence have been actively studied since they have high luminous efficiency.
However, a phosphorescent OLED is deficient in that high luminous efficiency cannot be obtained at high luminance regions, which is important in a practical device, since luminous efficiency decreases significantly as current density increases.
To solve this problem, an OLED having the structure shown in FIG. 4 has been developed. See, Advanced Materials, vol. 20, No. 21, p. 4189 (2008). As is shown in FIG. 4, the anode (indium tin oxide, ITO) 1, the hole transport layer 2 comprising a hole transport material (3DTAPBP), the first emission layer 3 comprising a first host material (TCTA) and a light-emitting material (FIrpic), the second emission layer 4 comprising a second host material (DCzPPy) and the light-emitting material, the electron transport layer 5 comprising an electron transport material (BmPyPB), and the cathode (LiF/Al) 6 are formed in this order. This OLED comprises two-layered emission layers. When a voltage is applied between the cathode and the anode, holes and electrons are injected from the anode and cathode, respectively, and are recombined within the emission layers to produce excitons, leading to light emission.
FIGS. 5A and 5B show recombination rate and exciton concentration for an OLED comprising two-layered emission layers (hereinafter, referred to as a double emission layer) shown in FIG. 4, and for an OLED comprising a single emission layer.
In general, triplet excitons produced in a phosphorescent material are annihilated without light emission, which is in proportion to the square of the concentration. Thus, when the current density is increased such that the exciton concentration is increased, resulting collisions of excitons decrease luminous efficiency.
In the case of the single emission layer shown in FIG. 5B, recombination of holes and electrons is caused in either the anode or cathode side of the emission layer, depending on the relationship between the mobilities of holes and electrons in the host material. Therefore, the excitons produced through recombination can diffuse in only one direction. This increases the maximum exciton concentration and rate of non-radiative annihilation and so decreases the efficiency.
In the case of the double emission layer shown in FIG. 5A, recombination is caused in the interface between the first emission layer 3 and second emission layer 4, that is, in the central portion of the double emission layer, because of the energy gap between the two host materials. Thus, excitons can diffuse from the central portion of the double emission layer in two directions: toward the anode and cathode sides. Therefore, in comparison with the single emission layer, it can lower the maximum exciton concentration resulting in decrease of the rate of non-radiative annihilation, thereby achieving the improvement in the luminous efficiency.
In a phosphorescent OLED having a double emission layer, high luminous efficiency has been obtained at high luminances, but it is still not satisfactory. This is because there are emission centers in the interface between the two emission layers each comprising different type of host material, where problems such as trapping of electrons and holes and non-radiative deactivation of excitons are caused, which tends to lower the luminous efficiency. Another problem is that concentration of recombination and emission centers in a narrow region of the interface, though it is in the central portion of the emission layer, raises maximum exciton concentration and lowers the luminous efficiency and lifetime.